Running VMs but its way too rich

GregT

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Hi Guys, sorry for being away...

Okay I finally got a pair of Mukuni 34mm VM carbs. they came with 260 mains 35 pilots. Swapped out the mains with 135# and pilots with 22.5#. It's still too rich everywhere. Any suggestions? Oh yea stock exhaust and Mike's XS Pods...
 
I'd say 34's are wayyy too big for a stock engined xs400. I hope you can get them to work but I think its going to be very difficult to get them to work well across the whole rev range. Im about to start tuning my brand new Vm30's just waiting on new jets. Although vm34's are technically the same size as the stock bs34 cv carbs, Vm's have way less internal restriction and flow a lot more as they're round slide carbs and not vacuum operated hence why you should probably go a little smaller. One thing people complain about when changing from stock to roundslides is thrttle response and bogging at 1/4 throttle. I think having carburetors that are too big are only going to accentuate that problem
 
Pretty sure theyre the same spigot diameter as 34's. They can clamp down but im going to put a strip of bicycle tube and some sealant to clamp down a little better
 
thanks but for now Im stuck with the 34s. So Ill have to figure out the jetting.What jetting are you using in your 30s?
 
I found a jetting guide for a kz400 somewhere that im going to start off with and then tune from there, vm34 jetting would be completely different. Maybe you should start off with a jetting guide for xs650s and lean it out by a third. But as I said I really dont think you will ever get it running right with carbs so big
 
If I was you I would sell the 34s. Shouldnt be hard to find a buyer. Then buy 30's, their only 75 each brand new from amazon. This is what im gonna start from, should get it running atleast
**change supplied main from 250 to 230 (hex jets)
**change pilot from 40 to 27.5
**change throttle valve cutaway (slide) from 2.5 to 3.0 (if mid range is rich if not ill leave it)
*Jet Needle : stock 6F5 on #4 (1 clip richer)
keep needle jet stock P-5
Keep air jet 2.0


Main will probably be too rich but low and mid range should be close. Now im just waiting on my other carby too get delivered hopedully sometime this week
 
I got it! The ID for both BS & VMs r identical. Right now they have 22.5 pilots, 135 mains and using the needle jet and needle that comes with them. Needle clip in top (leanest) position. Runs great no hesitation anywhere if any thing I might replace needle with the next leaner one in order to have more adjustment. So the 34s will work! Thanks for your help it got me thinking. Oh yea the air screw is 1 turn out. I might open up the exhaust a little. We'll see.
 
Sorry for the delay but I misplaced all my paper work. Of course, I found it right where I left it, duh! Here the specs: Mikuni VM 34-275 (Round Slide), Mains=135, Pilot= 22.5, Jet #159 Q-2, Needle #6DHA( clip in top position), Slide #2.5. It does a little hesitation at 1/4 throttle when not under load. I believe going to the next leaner needle and dropping the clip would clean this up
 
You mind posting up a video? I have VM30s and can not get it to run right with primary style emulsion tubes. When I had the bleed type I could get it to run decent to were I wasnt scared that I would get stuck somewhere. Still ran very rich, fouled plugs all the time until I switched to iridium plugs and could not pull on the throttle too quick or would bog out and finally pick back up. Now with primary style tubes it pops and cracks at cruising speeds. WOT its a blast to drive! To me it sounds like my needle isnt right but I've tried everything; plugs always black and never really makes a difference. I just ordered VM26s through speedmoto as I think the vm30s still too big. However I dont claim to be anywhere near a VM carb expert
 
What specs are you runnig Campbell45? I was actually going to PM you to see how you ended up because ive seen your threads around the internet when I was debating getting vm30's haha. I think 26's would be way small and I dont think you could get them to work with stock boots. Stock diameter of the boots is supposed to be 44mm. Vm30's spigot are 40mm and only just work and Vm26's are 35mm
 
Im getting a new throttle housing in a few days, so after I make up my new 2 into1 cable ill get to testing my Vm30s using the specs I posted earlier and post back here with my findings
 
I just got the vm26s today. Swapping them over right now I'll let you know how it goes. I'm not sure how they will perform as the difference between the two are pretty big. I know the carbs will have more air velocity though!

I will also post of specs with what I have once I get them off. Glad to see there are other people putting VMs on. For awhile I felt lonely
 
Okay for the VM30s I had:

Main: 150

Pilot: 30

Needle: 6F5

Slide: 2.0

Air screw: 1 turn out

Needle position: leanest (which it did not like, go to middle. I had it at middle, shop changed it to this :shrug:)

The bike would start up fairly easily and would go down the road but there was alot left on the table that needed improvement. Throttle response sucked, plugs fouled out easily, and it sounded like shit going down the road.


I now have the VM26s in and have yet to get them dialed in. However I like what I am seeing. Did not start up with choke on but burped a couple times. Choke off and started up first kick and idled beautifully. Could not give it any throttle or would die. Turned choke on for a split second then back off and was able to go through full throttle range with good throttle response for a short while before reverting back to no throttle. To me this spells out that my pilot is lean. Will mess with it more later but I think the Vm26 will work well.

VM26 jetting so far is:

Main: 150

Pilot: 37.5

Needle: 5F21 (middle position)

Slide: 2.0

float height: 17mm

Air screw: 1 turn out

Air Jet: .5
 
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Well Timbuz I hope you havent ordered the VM30s yet because I just went on a ride with the vm26 and I cant stop grinning. Throttle response is absolutely incredible, hardly any lag compared to my VM30s. Granted I didnt get the 30s tuned in very well but everything I did I was either too rich or wouldnt start up easily. Thus why I had my suspicion of too much air.

The vm26s I still need to tune in as it doesnt like to idle down and the transition from off idle to mid likes to pop and crack which I think is a lean pilot. Will play with it some more once I get some jets. But I would like all of you to know that I am extremely impressed with the 26s :bike:
 
Good to hear! Ive already got the 30's, I think theyll be pretty well matched to the bike once tuned, tons of people have tuned them right for cb350s so they definitely wont be too big. How do the 26s fit? Did you get aftermarket carb boots?
 
So I got the VM30's on quickly today and started it up. Only took a couple of kicks. Havent synced them or adjusted the idle properly yet. Just need to put some rtv around the carbs to get a better seal as they only just clamp down and the throttle was hanging a little bit from the air leak. But other than that it revved all the way out. Almost shook the house down haha. I think the settings ive got them at will be a real good starting point. Hopefully road test it in a few weeks when I sort a few more things out, heres a vid from today haha I was pretty excited
 
Thats great TimBuz! Have you taken her for a drive yet? Whats your jetting set up right now? Also are you sure you hanging throttle is an air leak?


I'm going round and round with my vm26s. I swear they should send an extra gasket set because you have to take these apart so many times to make adjustments :banghead:
 
Yeah I havent got gaskets on the rubber boots and I need to get some sealer around the carbs or rubber tube so the clamps will bite down better so atm it takes a little while for revs to come back down. I havent ridden it yet still gotta do a few things so might be a month away. Jetting specs are what I posted above but ive got the 2.5 slides in. Only thing I changed from factory specs are 220 main jets and 27.5 pilots and 1 clip richer on the needle . After I road test it and do some plug chops ill fine tune it but after reving it out the other day I definitely think im at a point where it would be rideable. I still think 26's would be too small in the end, they might give you some more throttle response on the low end but will surely restrict your top end. Try your 30's with the specs I posted looks like you could road test sooner than me anyway
 
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